This is most often an issue with a site setting.  There must be a default email address in the correct field:

i.e. info@your-org.org.

Another option could be an expired Password:

If you have an instance with only one person in an office not getting the pin, but everyone else is.  Have another Admin manually change the password to get the account “active” again (possible expired password), and then send the password reset.  Note, when you change the reset it will take you to a Admin login with a pin request, and will send the pin request to the email on record.  Have them ignore that one (if the Admin is in another office) and it should generate a new one, once they reset their password themselves.

Contact Support and reference this solution number, and they can get this resolved for you quickly.